The present invention relates generally to ignition systems in spark ignited engines, and more particularly relates to such systems in lean burn engines in which the excess air factor xcex is greater than 1.4.
Industry has developed various techniques using ionization signals for detecting abnormal combustion conditions such as misfire, knock, and approximate air/fuel ratio for stoichiometric engines. Free ions present in the combustion gases are electrically conductive and are measurable by applying a voltage across an ionization probe. Alternatively, the voltage is applied across the electrodes of a spark plug after the spark plug has ignited the combustion mixture. The applied voltage induces a current in the ionized gases that is measured in order to provide an ionization signal at a magnitude that is readable by a controller such as a microprocessor. The ionization signal is used in the control of the engine. For example, in U.S. Pat. No. 6,029,627, ionization signals and a single O2 sensor in the exhaust are used to control the air/fuel ratio in engines to achieve stoichiometric operation. This technique uses the O2 sensor to achieve stoichiometry of the overall mixture of the engine and then equalizes the amplitude or location of the first local peak of the ionization signal in each individual cylinder. Another technique disclosed in U.S. Pat. No. 5,992,386 performs a frequency analysis of the ionization signal to detect abnormal combustion conditions such as knock. These systems integrate the ionization signal and compare the magnitude of the integrated signal to the magnitude of the integrated signal of a normal combustion event. The abnormal combustion condition is detected if the magnitude of the integrated signal is above a threshold level, which is set above the magnitude of the integrated signal of a normal combustion event.
One of the drawbacks of stoichiometric engines is the emission of pollutants. With fixed engine timing and load, the NOx emissions level of a typical gas engine is dependent upon the air/fuel ratio. Near a chemically correct (i.e., stoichiometric) ratio, the NOx emissions peak and then drop significantly as the amount of excess air is increased. Maintaining a stable combustion process with a high air/fuel ratio is difficult to manage. As a result, conventional spark-ignited gas engines typically operate near the stoichiometric air/fuel ratio and depend upon exhaust after treatment with catalytic converters to reduce the NOx emissions.
Government agencies and industry standard setting groups are reducing the amount of allowed emissions in an effort to reduce pollutants. As a result, industry is moving towards using lean burning engines to reduce emissions despite the difficulty of maintaining a stable combustion process in lean burning engines. By using more air during combustion, turbocharged lean-burn engines can enhance fuel efficiency without sacrificing power and produce less nitrous oxide pollutants than conventional stoichiometric engines.
Ionization sensing has not been utilized to any significant extent in these lean burn engines. Because of the lean nature of the mixture, the ionized species concentration, including NOx, is much less than at stoichiometric conditions. As a result, the ionization signal is of very low intensity and has great variability. The techniques developed using ionization signals for stoichiometric operation are unsuitable for lean burn operation and do not work. For example, the ionization signals of some lean burn engines are sufficiently variable and are low enough in magnitude that integrating the signal cannot be done reliably due to a number of factors. These factors include higher levels of noise relative to the ionization signal magnitude, the variability of the ionization signal, and the low magnitudes of the overall signal and the resultant integrated signal.
In view of the foregoing, an object of the present invention is to provide a system and method to monitor and control the combustion quality of a lean burn engine using ionization signals.
The foregoing objects are among those attained by the invention, which provides a system and method to monitor the combustion quality of a lean burn reciprocating engine and includes an ionization module for measuring a succession of ionization signals of the lean burn reciprocating engine for successive cycles of a running engine and processing a plurality of related ionization signals for signal stability, identifying, using an initial current level for all of the signals, a start point of the ionization signal and a peak for the resultant ionization signal, associating a geometric parameter with the resultant ionization signal which relates the start point of the ionization signal to the peak for the resultant ionization signal, and compares the geometric parameter against a reference geometric parameter related to a desired combustion quality relating to xcex greater than 1.4. It also includes an air/fuel module in communication with the ionization module. The air/fuel module adjusts a control parameter of the lean burn reciprocating engine such that an error difference between the geometric parameter and the reference geometric parameter is minimized.
In another embodiment, the ionization module is coupled to each of the plurality of cylinders. The air/fuel ratio module adjusts a control parameter of the lean burn reciprocating engine for each cylinder independently based upon the geometric parameter corresponding to the respective cylinder.
A method for analyzing and controlling the combustion quality in a lean burn reciprocating engine is also disclosed. The method includes receiving a succession of ionization signals of the lean burn reciprocating engine for successive cycles of a running engine and processing a plurality of related ionization signals for signal stability. The method includes the steps of identifying, using an initial current level for all of the signals, a start point of the ionization signal and a peak for the ionization signal, associating a geometric parameter with the ionization signal which relates the start point of the ionization signal to the peak for the ionization signal; comparing the geometric parameter against a reference geometric parameter related to a combustion quality in lean burn engines operating with a xcex greater than 1.4, and outputting an indication of the combustion quality. The indication of the combustion quality is used to adjust a control parameter of the engine such that an error difference between the geometric parameter and the reference geometric parameter is minimized.
The geometric parameter in one embodiment is a slope of a line from the starting point of the resultant ionization signal and passing through the peak of the resultant ionization signal. Alternatively, the geometric parameter is an aspect ratio of a box having a lower left corner at the starting point of a resultant ionization signal, a top at the peak of the resultant ionization signal, and a right side at a percentage of the peak of the resultant ionization signal.